Improving Home Appliance Safety with IEC StandardsDecember 01, 2020 by Chad Solomon, Microchip Technology
This article explores the IEC 60730 Class B standard for functional safety to address both mechanical and electrical design in appliances. Learn what the standard entails and controllers to help meet these standards.
Numerous appliances simplify daily or periodic activities in the home and help users perform them consistently and with minimal stress. When they are working properly, they are great. However, if a failure occurs, some appliances can become quite unsafe and cause major problems including fires.
To ensure that safety is designed into appliances, the International Electrotechnical Commission (IEC) created the IEC 60730 Class B standard for functional safety to address both mechanical and electrical design that Underwriters Laboratories has approved as well. Providing a safer user experience for ranges and cooktops as well as laundry equipment and more, a new touchscreen integrated circuit (IC) has been designed with built-in functional safety that has been certified to meet this standard.
Preventing Serious Appliance Problems
The National Fire Prevention Association (NFPA), an international nonprofit organization, in its NFPA Research report states that ranges or cooktops were involved in 62% of the reported home cooking fires. In fact, the report specifically identifies, “Unattended cooking was the leading cause of cooking fires and casualties.”
Also, according to NFPA Research, ranges or cooktops are the leading cause (46%) of deaths caused by cooking equipment from 2013 to 2017. Another source, the U.S. Fire Administration (USFA), part of the U.S. Department of Homeland Security’s Federal Emergency Management Agency (FEMA), reports newer data from 2018 (only for the U.S) but still has cooking as the leading cause of residential fires, see Figure 1.
Figure 1. Cooking was the main cause of residential building fires resulting in injuries in the U.S. in 2018. Source: U.S. Fire Administration.
Since the USFA’s National Fire Incident Reporting System (NFIRS) was introduced in 1999, reported home cooking fires have risen steadily, in spite of the significant technical advances that have been made in numerous other areas. Clearly, addressing the safety aspects of ranges and cooktops should be a goal of appliance manufacturers to help reduce these numbers.
Safety Regulations for Functional Safety in Appliances
Recognizing the need for built-in safety in appliances, the International Electrotechnical Commission (IEC) developed IEC 60730-1 that applies to automatic electrical controls “for use in, on, or in association with equipment for household and similar use.” While initially released in 1986, the document has had periodic upgrades and amendments with the most recent occurring in April 2020. Underwriters Laboratories (UL) has put its seal of approval on the standard as well.
The rationale behind functional safety is that all electronics and mechanical systems ultimately will fail. Since failures cannot be designed out, the next best thing is to ensure that when something fails, it fails in a fail-safe manner. Three safety classes are identified in IEC/UL 60730 with Class A for those products that are not intended to be relied on for safety aspects and Class C for those that are intended to prevent special hazards.
In between these, the IEC/UL 60730 Class B or simply Class B classification includes major home appliances, such as cookers/stoves, washing machines, dryers, dishwashers, refrigerators, and freezers. Class B addresses software and control function intended to prevent hazards if a fault occurs in the appliance. It applies to thermal cut offs, automatic door locks and other functions that stop cooking and laundry appliances from operating if unsafe conditions arise. In both the USA and Europe, Class B certification is now mandatory on all cooking appliances with a self-cleaning function, and on laundry equipment for controls associated with door auto-locking mechanisms.
Existing Appliance Approach to Safety
To address the safety concerns in appliances both in the kitchen and the laundry room, historical solutions relied on the microcontroller (MCU) that interfaced to buttons, sliders, and wheels, whether they are mechanical or capacitive. The MCU had a software library to monitor the safety and health of the capacitive touch buttons. Today, to meet these standards, appliances use capacitive touchscreens with a separate capacitive touch button controlled by a separate MCU with a safety library on it.
Some buttons that are part of the user interface are particularly critical to enable safe operation of the appliance and to prevent hazards such as house fires. For example, cooking appliances with a self-cleaning function shipping into the Americas or Europe are required to support two-touch on and one-touch off operation of the self-cleaning mode. It is important to note that self-cleaning ovens reach dangerously high temperatures of 900oF (500oC). Items stored in the oven can catch fire during this mode, so IEC/UL 60730 Class B mandates that appliances with this feature must support two-touch on and one-touch off operation.
The rationale behind this requirement is that a second touch to turn on self-cleaning mode is used to encourage the user to double-check that self-cleaning mode can safely start. With a single touch, for example, after the user smells smoke, the stop or cancel button is used to end this mode. Safe operation of the stop button is critical to the safety of the user. As a result, the button must function reliably and accurately in all environments. Dangerous fires can be avoided by helping to end the self-cleaning mode as quickly as possible without multiple touch interactions.
Monitoring the single analog peripheral is rather straightforward and easily done by the MCU. The peripheral monitors a few buttons that are multiplexed together. One peripheral can scan a series of buttons. While this functionality is built into the microcontroller, the appliance manufacturer still has to develop and qualify the safety function and is liable for doing so.
A New Approach to Appliance Safety
Even though functional safety has been implemented in appliances for many years, there is an option to satisfy this requirement now by applying classic safety principles to a modern touchscreen interface. As touchscreens get implemented or upgraded, this is an ideal way to design-in Class B certification.
The ATMXT336UD-MAUHA1 maXTouch controller family with its unique safety-related features eliminates the need for a separate button, so shut off can now be accomplished by a “soft” button anywhere on the touchscreen. While the ability to eliminate the button and reduce cost and maintain the required functionality should be sufficient incentive to appliance manufacturers, the simplified interface should be more attractive to users as well. The integrated approach is simple and more intuitive. It is more convenient as well since it removes the clutter and cost of the separate buttons. Perhaps even more important, the shutdown function can be altered in the user’s natural language for globalization. See Figure 2.
Figure 2. By putting the emergency stop function on the touchscreen, the appliance manufacturer can allow the user to select the language they are most comfortable with for the stop function.
The smoke detector pioneered home safety decades ago. Unlike the smoke detector, the Class B functional safety touchscreen can both alert and take action. It performs these proactive functions before it senses smoke or something in the environment without requiring human interaction.
Let’s look at an example of a safety concern that the Class B touchscreen can solve without involving the cook. If a heavy pot is accidentally dropped on hot stove with inductive burners, in many cases, the thick glass would protect the burners, but the thin indium tin oxide (ITO) touch sensor underneath is more fragile and could break. This breakage could cause a loss of localized touch functionality in a certain region of the touchscreen or even worse, the entire sensor could fail.
The controller with Class B monitors, in real-time, the health of the touch sensor automatically. In the background, when the sensor is not scanning for touches, the IC scans the touch sensor itself looking for different types of failures. As soon as a failure is detected, such as a crack in the touchscreen sensor, the controller notifies the host central processing unit (CPU) so it can shut down the burners automatically. This will occur without requiring any human intervention providing fail-safe operation.
While this type of functionality has been possible previously, the host had to trigger or poll the touchscreen IC for input. Now, the IC performs this function itself and pushes the messages to the host whenever a problem occurs. A special I2C bus message or a general-purpose IO (GPIO) pin on the IC tied to an interrupt pin on the system host CPU automatically provides the alert to initiate a shutdown. The built-in and self-actuating safety functions provide the means to prevent some of the conditions that could lead to a kitchen fire. See Figure 3.
Figure 3. In a typical appliance application, the Class B touchscreen controller easily interfaces to a host CPU.
To pass Class B certification, several functional safety tests are performed by the touchscreen IC. One of the key tests is the memory test. The touchscreen IC has a small amount of RAM and Flash memory, sufficient to perform the required functions in the application. In an embedded environment, the closed system does not run any customer code. To meet the Class B certification, there are rules that determine how frequently memory tests have to be run. For example, a series of walking 1s and walking 0s run in the background as a pattern test to check for RAM failures on the IC. See Figure 4.
Figure 4. A walking 1s test for an 8-bit memory sequentially verifies each bit.
The process starts by testing the free RAM, validating that it is good. Since most of the small amount of memory is filled with data, a portion of the application code is then moved into the tested portion to free up another portion and test it. The process continues until all RAM is tested. This shuffling of the application code, while it is running, allows all the memory to be tested with sequential testing.
Managing this test alone is a reasonably complex process and it is all part of the Class B IC that is simultaneously reporting multi-finger touch events in real-time. At greater than 60 Hz, 60 times per second, the IC reports touches to the host while testing the RAM and testing the sensor in the background. Similar testing is performed to test the non-volatile flash program code storage memory cells.
CPU registers in the controller are required to be tested to ensure they are operating correctly. This is accomplished by reading and saving the current value of the CPU register and storing the data in another register that is not being tested. Then, the CPU register is toggled to make sure the new setting stays. After this, the initial values are restored. This checks to make sure that the CPU registers can be set and reset properly to the right values.
Internal clock tests make sure that the clocks are running properly. There are various clock trees inside the IC and the testing confirms that they are being divided up and running properly. This is not an exhaustive list of tests but provides an idea of the types of tests performed by the IC.
Another safety aspect of the new IC is that the I2C bus has had a major upgrade for the communications between the system host CPU and the touch controller. I2C communications on the touch controller make use of two mechanisms to ensure the integrity of any data sent to and from the device. The first mechanism uses a sequence number. In addition, for the first time, a cyclic redundancy check (CRC) has been added to the bus to check that data corruption (a bit error) has not occurred.
Because an appliance can be an electrically noisy environment, this ensures that the coordinates the host receives are reliable and have not been corrupted in transport. Also, since a sequence number has been added to the I2C messages, now, every time coordinates are sent to the host, the host can determine if it missed a data packet or packets and take appropriate action. With internet connectivity, the host could even be programmed to notify the user’s cellphone or smartwatch of a problem, even if the user is not at home.
As noted previously, with standard Class B safety tests, additional intelligence has been added to the touch controller to enable periodic self- and sensor-diagnostic functions to constantly monitor the integrity of the touch subsystem. These smart diagnostic features support a configurable heartbeat ("keep alive") signal output that can be sent to the host using a general-purpose IO (GPIO) output trigger.
One final safety item. With functional safety already qualified in the touchscreen safety chip, the user’s system-level qualification process is greatly simplified since additional software does not have to be written on the host CPU to manage the safety of the touchscreen. This should provide significant motivation to appliance manufacturers to investigate and implement this new capability.
Laundry Room Safety
The Class B touchscreen controller can handle other safety situations outside of the kitchen. For example, the NFPA states that U.S. fire departments responded to an estimated average of 15,970 home structure fires involving clothes dryers or washing machines each year. While there are a variety of causes for these fires, design for safety should make a significant dent in the total number.
In the laundry room, there are Class B requirements for washing machines. Laundry equipment with high-speed motors, especially front-loading machines, has a locking mechanism so the door does not inadvertently open while the machine is operating. The autolocking mechanism is an electromechanical device that ensures that when the machine is turned on, the safety feature is initiated. If the locking mechanism is controlled by a touchscreen, then a separate button, microcontroller, and sensor could be eliminated.
The manual door unlock override allows items to be added or removed after the washer or dryer has started, and needs to be safety-compliant as well. This is currently the law for laundry appliances in the U.S. and Europe, where Class B certification is required. The new alternative puts everything on the touchscreen - and it applies to washing machines and dryers because of the heating aspects. Also, in front-loading washers, the system needs to be drained before the door is opened to prevent flooding. Since it is one aspect of their life that they can exert some control over, people are increasingly concerned with safety.
While a specific region may require the Class B certification on a specific product, once an appliance manufacturer meets that requirement, it will sell the product with that functionality/capability into other regions of the world and promote the touchscreen safety functionality as a competitive advantage. This has proven to be very effective in the past. The approach enables a global model for Europe, the Americas, and the rest of the world, so the manufacturer only has to make one type of that class of appliance.
Towards Safer Appliances
Initiated by inputs from leading appliance suppliers, the customer-initiated Class B touchscreen controller’s design and development took several man-years to bring from concept to reality. It had customer feedback during the entire process to fine-tune the Class B capabilities. While it certainly is a feature that suppliers will want to offer and customers will want to have in high-end appliances, some appliance makers are already considering Class B safety for entry-level designs since all customers deserve the highest level of safety available.
Once appliance makers are familiar with this new capability, its associated cost reduction and have certified an appliance or appliances to meet the Class B standard, they may seek to consider apply the technique to machines/appliances where it is not mandatory to gain a marketing advantage and brand differentiation for an entire lineup of safety-conscious appliances for safety-conscious consumers. For safely interfacing appliances to humans, the Class B touchscreen controller is a safe choice for ovens, ranges, washing machines, and dryers as well as dishwashers, refrigerators, microwave/convection oven combos, and even range fume hoods.
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